A transmitter's, such as a base station's, output power can basically be increased in two alternative ways; either by introducing components that are capable of providing the desired output signal or by using low-power components that process the same signal and whose outputs are adjusted to the same level and combined with an output signal of the transmitter. The latter is often a more advantageous way due to potential savings in view of costs of the transmitter.
When summing two same transmit signals to increase output power, the delays and the phases of the transmitters have strict requirements. Without accurate delay and phase control, it is impossible to sum outputs of several transmitters efficiently. In prior art, separate transmitters whose signals are combined are calibrated either to a nominal delay or with regard to each other. Calibration, however, requires additional measurement equipment and is very difficult and time consuming. Furthermore, calibration performed at a certain moment cannot take into account the delay drifting of the transmitter, which is due to aging, for instance.
In a feed-forward transmitter, summation of signals is done before power amplifiers on small signal level. The summed signal is then divided between two power amplifiers and combined again after the power amplifiers. Because of differences in delay and the phase of the power amplifiers, delays of the power amplifiers must be controlled accurately to achieve successful power combining.
A digital predistortion transmitter is a transmitter where the distortion caused by a power amplifying step of a transmitter unit is feedback to digital form and compensated digitally by summing a digital opposite distortion element to the signal.
When aiming to increase of transmit power in a predistortion transmitter by combining, the logic known from a feed-forward transmitter is not applicable. That is, small signal summing and division of the sum signal to separate power amplifiers would make it impossible to determine the correction distortions needed for individual transmitter units.